What's is the function of an APU?

[Capt. Manuvar]

I would be grateful if any one could explain what an APU does and how it works. What device is used in light a/c instead of an APU? None of my aviation material (JAA PPL/FAA CPL books) has any info on this.
Thanks
Capt Manuvar

[seat 0A]

An APU is a small jet engine, mostly in the tail of an aeroplane.
Its main function is to supply electrical power and air for airconditioning and engine starting, while on the ground.
It can also be used in the air, dependant on a/c type. In a 737 it can supply both electrical power and airconditioning in the air, while on a 744, it cannot even be started in the air.

Smaller aircraft mostly use ground power units on the ground. These are just electrical power-suppliers.

[fritzi]

A APU (auxiliary power unit) is a generator that is installed on most pax aircraft. It provides electrical power to the aircraft if the engines are switched off and a GPU (ground power unit)is not connected. It may also be used if you have a engine faliure or generator faliure in-flight to provide electrical power.

The APU is in effect a small jet engine, which drives an electrical generator. Hot air tapped off the compressor can power the pneumatic system of the aircraft and is used for air conditioning and engine starting.

The APU is usually located in the tail.


Edit:
And the winner is seat 0A!
Beat me by a minute!

[ROB-x38]

In turboprops such as the ATR family, rather than installing an APU the prop brake can be used. This allows the engine and generators etc... to be running without the prop turning.

So electricity and bleed air (for engine starting, aircon etc...) are available with no APU, no ground power unit and no spinning prop. Nice feature.

[witchdoctor]

On aircraft such as the BAe146, many cruelly refer to the powerplants under the wings as 4 APU's.

[Capt. Manuvar]

Thanks for the replies. i've always wondered what the little nozzle on the tail of a lot of twin jets was all about.
Capt. manuvar

[JeroenC]

Since weight is a big consideration in A/C design and one could use the ground power to power the A/C on the ground, this basically means that the APU is a safety device, used for in-flight failures?

Pls correct if I'm wrong.

[cortilla]

I may be wrong but if memory serves me correctly, If an apu is installed on a larger a/c (don't forget i'm used to flying a pa-28), then the apu is used to start all the other systems. This is because the apu can usually be started using a starter motor. The apu uses the a/c's own batteries, whearas the main engines need bleed air to start (from the apu or from a ground source). This is because the apu is much smaller and therefore needs much less torque (hence much less energy) to get going. Once it's going it can get the main engines going as well. Therefore if there is a functioning apu on board the aircraft then much less ground handling is needed to get the a/c going. I can't see when an APU would be used for in flight emergencies. If all the engines fail then i would imagine the batteries and/or the RAT (ram air turbine) would take over the neccesary emergency functions, and not the apu.

[ROB-x38]

JeroenC

As seat 0A mentions, whether or not the APU can be used in-flight or not varies from aircraft to aircraft. Aircraft certified for ETOPS usually require an operative APU for, as you mention, safety/redundancy reasons.

For other aircraft the APU may not be approved for operation in-flight, it's main use being for engine starting, aircon etc... at airports without ground power.

So really it varies from aircraft to aircraft.

[terryJones]

Does the APU provide any thrust at all. I don't mean intentionally, but after all, there must be something coming out the back to produce a reaction that may be noticable.
TJ

[ROB-x38]

APU thrust

I suppose it must but the effects on performance would be negligible.

I've heard the trident used its APTU (Auxiliary Power & Thrust Unit) on takeoff when field / climb performance was borderline. Don't know any figures.

[Flight Detent]

Hi all,
No Boeing I've ever heard of takes the APU exhaust into account with regard to extra thrust.

A small point, the current crop of B737s have an APU installed, but the bleed air that is supplied for aircraft systems is extracted from an air compressor driven on the main drive shaft, sharing a common air inlet with the actual APU turbine compressor, unlike any other APU I've ever seen before.

The B727 APU is installed in the main wheel well, all others I believe are installed in the tail cone.

The late model B737s can start their APUs up to about 25,000 feet, and operate them right up to 41,000 feet, though only for electrics at those sorts of altitudes!

Cheers

[cumulusse]

hi there

the is some real help from the APU when you land at remote airports without any ground support,better have it running until engines start.

[allthatglitters]

"On aircraft such as the BAe146, many cruelly refer to the powerplants under the wings as 4 APU's."

Excuse me witchdoctor, but shouldn't that read 3 APU's and 1 overtime generator...

Rob-x38
As to the Trident, the models 1 & 2 had the APU above the #2 engine, just below the rudder. The Trident 3 series had the APU above the #2 engine intake and a pure boost engine (RB162 approx 6,000lbs thrust) above the #2 engine and below the rudder.

Oh what a combination of sound, 3 Spey's and an RB162 in close formation, shattering.

[Rhodie]

Just as an aside....

Thought I had to add:

Down here on the dark continent, we still fly freight in/out of the neighbours using some machines left over from WW2...

The DC4 also has an APU - ours is a Honda generator under the cockpit floor....

Cheers

R

[ROB-x38]

V interesting allthatglitters, thanks for the info.

Just to make sure i've got this right.... in this photo of a Trident 3 the APU is above where it says "Trident three" and the RB162 is the bulge on the fin with the red stripe on it correct?

And where is the intake for the RB162?